nv50/ir: Check for valid insn instead of def size

nv50/ir: Check for valid insn instead of def size

This fixes a null pointer dereference during the register allocation pass,
if a function had arguments.

Functions arguments get a definition from the function itself, a definition
which is therefore not linked to any instruction. If a value ends up having
a definition but no linked instruction, the register allocation pass doesn't
need to consider whether that value is generated by an instruction that
can only handle "short" registers (on nv50).

Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>